For Fire I don't think it really makes any difference. Although, in 5.2. the Mirror Image Frostbolt is getting buffed so we'll have to do some tests again. For Arcane it's definitely a dps gain because the images go to 6 Charges and don't spend mana.

Things may have drastically changed since I last looked at this glyph, so take what I say with a grain of salt. The glyph is a DPS gain for Arcane, DPS neutral for Frost, and a DPS loss for Fire. The reason for this was that the Fireball they cast ends up taking too long and deals less overall damage than if they just spammed FrB and Fireblast.

Things may have drastically changed since I last looked at this glyph, so take what I say with a grain of salt. The glyph is a DPS gain for Arcane, DPS neutral for Frost, and a DPS loss for Fire. The reason for this was that the Fireball they cast ends up taking too long and deals less overall damage than if they just spammed FrB and Fireblast.

False, for Fire they are a gain. Go to the dummies, and try one CD glyphed and one unglyphed and the glyphed one will do more damage (25-50k IIRC). Yeah it is minimal but still.